This invention relates to nuclear spin tomography (nuclear magnetic response (NMR) imaging) in general and more particularly to an improved shielding device for use in nuclear spin tomograph apparatus.
A magnetic device, which is part of apparatus for use in nuclear spin tomography, having several magnet coils which enclose an interior suitable for containing a body to be examined and generate therein an at least largely homogeneous magnetic field, and having a cylindrical shielding device which surrounds the coils; consists of ferromagnetic material; has a part which is at least largely shaped like a cylindrical surface; and has, at each end face, a disc-shaped part with a central opening with a predetermined radius with respect to the cylinder axis is described in European Pat. No. 67 933 A1.
In the field of medical diagnostics, image-forming methods have been developed, in which an image similar to an x-ray tomogram is constructed by computer or measurement analysis of integral resonance signals of nuclei such as protons from the three-dimensional spin density and/or relaxation time distribution of a body to be examined. The corresponding method is also called zeugmatography ("Nature", vol. 242, 1973, pages 190 and 191).
Since, as is well known, the nuclear resonance signal increases with an increasing magnetic base field, a basic field as strong as possible is desired by systems in nuclear spin tomography (nuclear magnetic resonance systems), which field had sufficient homogeneity over a measuring range. On this basic field is superimposed a stationary and/or pulsed gradient field. In addition, a high-frequency field oriented perpendicular to the base field must be provided. The dimensions of the coils needed for forming a magnetic coil arrangement must be matched to the dimensions of the body to be examined so that the body can be inserted without difficulty into the measuring space within an interior surrounded by the coils.
The strong magnetic field of such a magnetic coil arrangement, however, should be contained, substantially only within the measuring space; it should not spread, as far as possible, in the form of a stray field, into the vicinity of the rest of the apparatus where it can lead, for instance, to interference in certain electronic circuits or also can exert certain undesirable forces on ferromagnetic movable parts. Therefore, separate shielding devices are required in order to reduce the stray field outside of the magnet coil arrangement.
Such a shielding device is disclosed, for instance, in European Pat. No. 67 933 A1, mentioned at the outset. This device, which encloses a magnet coil arrangement with a cylindrical, normally conducting field coil, contains a closed cylindrical surface of iron which surrounds the coil. At the end faces of this cylindrical surface, washers of iron with circular central holes are attached, the diameter of which holes is determined substantially by the dimensions of the body to be examined and the required homogeneity of the magnetic field. In addition, the magnet coil arrangement is rigidly fastened to the inside of the shielding device designed in this manner.
Since the known shielding device comprises a closed cylindrical surface, it can be used only for normally conducting, but not for superconducting magnets. For, in such superconducting magnets, lateral openings such as for the so-called turret of the required cryostat as well as for pump nozzles are required. In addition, there is no possibility of adjusting the magnet coils for correcting gradient errors with this device.
It is therefore an object of the present invention to improve the above-mentioned magnetic device with a shielding device in such a manner that the mentioned difficulties are at least largely corrected and where sufficiently high field homogeneity is preserved in the measuring space.